Sometimes we miss the big picture.
Here then, for your approval and disapproval are comparisons between closest kin found by the Nesbitt (2011) tree (Fig. 1) versus those found by the large reptile tree.
The Split-Up of the Dinosauria is today’s topic.
The Dinosauria (however it originated) has been traditionally split up into Saurischia (Theropoda + Sauropodomorpha) and Ornithischia. Nesbitt (2011) also found this traditional branching with the addition of the Silesauridae as an outgroup and Marasuchus as their common ancestor.
The Nesbitt (2011) Tree as Told by Skeletons
Since the Dinosauria was recovered as a single clade by both the Nesbitt study and the large reptile tree, the “strange bedfellows” here will be more difficult to see from small drawings such as these (Figs. 2-3). Back in 2011, I might have come up with a very similar tree. However, a wealth of basal dinosaurs have come to light since 2011, shifting the branches a wee bit.
Results of the Large Reptile Tree
In the large reptile tree Gracilisuchus and Turfanosuchus are the outgroup taxa. They nested together in the Nesbitt (2011) tree as in the large reptile tree, despite their many differences. Together they would have nested much closer to dinosaurs in the Nesbitt (2011) tree, except for his unfortunate inclusion of pterosaurs and lagerpetids. Trialestes (Fig. 3) was not tested by Nesbitt (2011) and that’s too bad, because it is a key taxon.
Herrerasaurus and the theropods, including Marasuchus, nested closer to Trialestes than any other dinosaurs, all of which were more derived plant-eaters. Pampadromaeus (Fig. 3) nested at the base of the Phytodinosauria. Pisanosaurus, always considered a basal plant-eater, nested at the base of the poposaurs, now nested within the Dinosauria with their redeveloped calcaneal heel. Massospondylus (Fig. 3) nested at the base of the sauropodomorpha. Daemonosaurus, another new taxon that Nesbitt considered a strange theropod, but did not include in his archosaur paper, nested instead at the base of the Ornithischia.
The Importance of Pampadromaeus
Prior to the inclusion of Pampadromaeus, Daemonosaurus nested as the transitional taxon into the phytodinosauria. Someday I hope we’ll see what its post-crania looks like. I anticipate an unusual transitional pelvis, not quite ornithischian in morphology. Panphagia might demonstrate the first stages of this. For now that short round skull and large premaxillary teeth are traits basal phytodinosauria share in common. Later forms independently developed longer longer skulls and other various shapes.
Pampadromaeus apparently preserves only cervical ribs, not cervical centra. These ribs overlap shorter centra on Herrerasaurus, so Pampadromaeus may not have had such a long neck as originally envisioned. The large reptile tree nesting of Pampadromaeus matches that of the original study (Cabiera et al., 2011).
Timing Is Everything
Unfortunately Nesbitt (2011) was published prior to the publication of taxa bridging the theropoda and the rest of the Dinosauria. Fortunately the web can be updated daily as new discoveries shift branches this way and that.
As always, I encourage readers to see specimens, make observations and come to your own conclusions. Test. Test. And test again.
Evidence and support in the form of nexus, pdf and jpeg files will be sent to all who request additional data.
Nesbitt SJ 2011. The early evolution of archosaurs: relationships and the origin of major clades. Bulletin of the American Museum of Natural History 352: 292 pp.
Sues H-D, Nesbitt SJ, Berman DS and Henrici AC 2011. A late-surviving basal theropod dinosaur from the latest Triassic of North America. Proceedings of the Royal Society B, published online
Cabreira SF, Schultz CL, Bittencourt JS, Soares MB, Fortier DC, Silva LR and Langer MC 2011. New stem-sauropodomorph (Dinosauria, Saurischia) from the Triassic of Brazil. Naturwissenschaften (advance online publication) DOI: 10.1007/s00114-011-0858-0
Martínez RN and Alcober OA 2009. A basal sauropodomorph (Dinosauria: Saurischia) from the Ischigualasto Formation (Triassic, Carnian) and the early evolution of Sauropodomorpha (pdf). PLoS ONE 4 (2): 1–12. doi:10.1371/journal.pone.0004397. PMC 2635939. PMID 19209223. online article